In general, nematic liquid crystal displays (LCD) are operating on the basis of dielectric coupling, i.e. the coupling between dielectric anisotropy (Δ∈) of the liquid crystal and an applied electric field which gives rise to an electro-optic response. This response is quadratic with the applied field, i.e. it is not polar, and arises from the switching of the liquid crystal molecules by the field. In conventional nematic LCDs, the switching of the liquid crystal molecules takes place in a plane containing the direction of the applied electric field which means that an electric field applied across a liquid crystal sandwich cell will switch the molecules out-of-plane, i.e. in a plane perpendicular to the cell substrates. This kind of switching, however, gives an electro-optic response having a contrast strongly dependent on the viewing angle. Moreover, the total time of the response (τ), i.e. the switching time, which is the sum of the rise (τr) and the fall (field-off) (τf) time, is usually not short enough for displaying moving images.
On the other hand, LCDs having an interdigitated electrode pattern (generating an in-plane electric field) deposited on the inner surface of one of the substrates, exhibit in-plane switching (IPS) of the optic axis and thus provide images whose contrast is less dependent on the viewing angle. In the improved version S-IPS (super in-plane switching) a herringbone electrode structure is utilised. Nonetheless, the switching time of the displays operating in IPS-mode is not short enough for generating high quality moving images.
In-plane electric fields could also effectively be generated by comb-like electrode structures generating a fringe electric field. However, also in these so-called fringe field switching (FFS) devices, dielectric coupling is generally utilised and the problem with the long field-off time mentioned above is thus not solved.
It should be mentioned that in the cases described above, the field-off time (τf) does not depend on the magnitude of the applied electric field whereas the rise time (τr) does. Hence, while the rise time could be efficiently controlled by the electric field, the field-off time is field-independent. It depends only on the cell characteristics, such as cell gap, as well as on the liquid crystal materials parameters, such as viscosity and anchoring strength to the solid substrates.
Another known method for switching a nematic liquid crystal between different optical states utilises the linear coupling between flexoelectric bulk polarization of an initially deformed nematic liquid crystal and an applied electric field (“Flexoelectrically controlled twist texture in a nematic liquid crystal”, Dozov et al, J de Phys Lett, 43 (1982), L-365-L-369); and “A novel polar electrooptic effect in reversely pretilted nematic liquid crystal layers with weak anchoring”, Komitov et al, Proceedings of 3rd International Display Research Conference, October 1983, Kobe, Japan).
WO 2005/071477 describes a liquid crystal device comprising a flexoelectric liquid crystal bulk layer, wherein an inhomogeneous electric field in a direction substantially parallel to the substrates is generated by an interdigitated electrode pattern. It is preferred that the average polarization direction in a direction parallel to the substrates in field-off state is orthogonal to the direction in which an electric field is to be generated. In this case, both the rise and the fall times become field-dependent and the total response time is thereby decreased.
A ferroelectric liquid crystal (FLC) display device including a comb-like electrode is also known (JP 10-161128).
In-plane switching of a nematic liquid crystal by an electric field applied across the cell substrates has been realized recently by using an electrically commanded surface (ECS). The published international patent application No. WO 00/03288 describes the so-called ECS principle.
According to the ECS principle, a separate thin chiral smectic liquid crystalline layer, preferably a ferroelectric (chiral smectic C phase, SmC*) liquid crystalline polymer layer, is deposited on the inner surface(s) of one or both of the substrates confining a liquid crystal bulk material in a conventional sandwich cell.
The chiral smectic liquid crystalline polymer layer acts as a surface-director alignment layer imposing a planar or substantially planar alignment on the adjacent liquid crystal bulk material. More specifically, when applying an external electric field across the cell—and thereby across the surface-director alignment layer—the molecules in the separate chiral smectic liquid crystalline layer will switch. The change of the dynamic surface-director alignment layer in response to the electric field is referred to as the “primary surface switching”. This primary surface switching results in its turn, via elastic forces (steric coupling), in a switching of the preferred molecular orientation within the bulk volume of the liquid crystal bulk material confined between the substrates. This secondary switching is referred to as the “induced bulk switching”. This induced bulk switching is an in-plane switching. Thus, the molecular switching in the dynamic surface-director alignment layer will be transmitted into the bulk volume via elastic forces at the boundary between the separate surface-director alignment layer and the bulk layer, thus resulting in a relatively fast in-plane switching of the bulk volume molecules mediated by the dynamic surface-director alignment layer.
The chiral smectic liquid crystalline layer, i.e. the dynamic surface-director alignment layer, may be a synclinic or anticlinic chiral smectic, e.g. smectic C (SmC* or SmCA*), material or a chiral smectic A (SmA*) material, including so-called random SmC*. Thus, the response of the dynamic surface-director alignment layer to an applied electric field may be ferroelectric, antiferroelectric or paraelectric, respectively.
The published international patent application No. WO 2003/081326 describes a liquid crystal device comprising a liquid crystal bulk layer and chiral dopants inhomogeneously distributed in the bulk layer as a result of being permanently attached to at least one surface, said dopants thereby inducing a spontaneous polarisation in a sub-volume of the bulk layer adjacent said surface. The response of the bulk layer within said sub-volume to an electrical field applied over the bulk layer may be ferroelectric, antiferroelectric, or paraelectric.
The use of an ECS layer/sub-volume in a liquid crystal device provides a fast in-plane switching and a comparatively high image contrast. However, it would be desirable to improve the contrast even further. Furthermore, the required voltages are rather high.